Efficient droplet activation of ambient black carbon particles in sub-urban environment

Abstract. The cloud condensation nuclei (CCN) activity of black carbon particles (BC) importantly determines their impacts on cloud microphysics and atmospheric lifetime. This process is crucially determined by the amount of more hygroscopic coating materials BC can acquire during aging process. It remains a challenge for ambient measurements to capture this process and link this with CCN activity of BC. Here by using coupled measurements of size-resolved number concentrations of CCN at configured water supersaturation (SS) and BC-containing particles (BCc), we are able to directly monitor the droplet activation diameter (D50) and activation fraction of BCc (F_act,BC) in ambient environment. The number concentration of BCc was found to peak at diameter 180–210 nm after acquiring coatings, larger than that for all particles (50–150 nm). This led to that the initially smaller BC can be enlarged and become larger and more hygroscopic, even having higher CCN activities than other particles. The F_act,BC increased from 42 % to 69 % in number and from 67 % to 85 % in mass, as SS increased from 0.1 % to 0.2 %, but tended to reach a plateau when SS>0.2 %.Notably, F_act,BC and D50 linearly correlated with equivalent photochemical age, at a rate of +2 % per hour and -3 nm per hour, respectively. The results suggest BCc from anthropogenic sources can readily serve as CCN at a relatively low SS, and over half of the BC population can be activated in a few hours. This means the surface-sourced BC can efficiently incorporate into clouds and may exert important indirect radiative impacts.